Re: [RFC PATCH v2] dma-buf: Add a capabilities directory

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On Thu, May 26, 2022 at 12:40 PM Simon Ser <contact@xxxxxxxxxxx> wrote:
To discover support for new DMA-BUF IOCTLs, user-space has no
choice but to try to perform the IOCTL on an existing DMA-BUF.
However, user-space may want to figure out whether or not the
IOCTL is available before it has a DMA-BUF at hand, e.g. at
initialization time in a Wayland compositor.

Add a /sys/kernel/dmabuf/caps directory which allows the DMA-BUF
subsystem to advertise supported features. Add a
sync_file_import_export entry which indicates that importing and
exporting sync_files from/to DMA-BUFs is supported.

Signed-off-by: Simon Ser <contact@xxxxxxxxxxx>
Cc: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Bas Nieuwenhuizen <bas@xxxxxxxxxxxxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
---

Oops, I forgot to check in new files after spliting a commit.
Fixed.

This depends on:
https://patchwork.freedesktop.org/series/103715/

 .../ABI/testing/sysfs-kernel-dmabuf-buffers   | 14 +++++
 drivers/dma-buf/Makefile                      |  2 +-
 drivers/dma-buf/dma-buf-sysfs-caps.c          | 51 +++++++++++++++++++
 drivers/dma-buf/dma-buf-sysfs-caps.h          | 16 ++++++
 drivers/dma-buf/dma-buf-sysfs-stats.c         | 13 +----
 drivers/dma-buf/dma-buf-sysfs-stats.h         |  6 ++-
 drivers/dma-buf/dma-buf.c                     | 43 ++++++++++++++--
 include/uapi/linux/dma-buf.h                  |  6 +++
 8 files changed, 133 insertions(+), 18 deletions(-)
 create mode 100644 drivers/dma-buf/dma-buf-sysfs-caps.c
 create mode 100644 drivers/dma-buf/dma-buf-sysfs-caps.h

diff --git a/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers b/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
index 5d3bc997dc64..682d313689d8 100644
--- a/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
+++ b/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
@@ -22,3 +22,17 @@ KernelVersion:       v5.13
 Contact:       Hridya Valsaraju <hridya@xxxxxxxxxx>
 Description:   This file is read-only and specifies the size of the DMA-BUF in
                bytes.
+
+What:          /sys/kernel/dmabuf/caps
+Date:          May 2022
+KernelVersion: v5.19
+Contact:       Simon Ser <contact@xxxxxxxxxxx>
+Description:   This directory advertises DMA-BUF capabilities supported by the
+               kernel.
+
+What:          /sys/kernel/dmabuf/caps/sync_file_import_export
+Date:          May 2022
+KernelVersion: v5.19
+Contact:       Simon Ser <contact@xxxxxxxxxxx>
+Description:   This file is read-only and advertises support for importing and
+               exporting sync_files from/to DMA-BUFs.
diff --git a/drivers/dma-buf/Makefile b/drivers/dma-buf/Makefile
index 4c9eb53ba3f8..afc874272710 100644
--- a/drivers/dma-buf/Makefile
+++ b/drivers/dma-buf/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 obj-y := dma-buf.o dma-fence.o dma-fence-array.o dma-fence-chain.o \
-        dma-resv.o
+        dma-resv.o dma-buf-sysfs-caps.o
 obj-$(CONFIG_DMABUF_HEAPS)     += dma-heap.o
 obj-$(CONFIG_DMABUF_HEAPS)     += heaps/
 obj-$(CONFIG_SYNC_FILE)                += sync_file.o
diff --git a/drivers/dma-buf/dma-buf-sysfs-caps.c b/drivers/dma-buf/dma-buf-sysfs-caps.c
new file mode 100644
index 000000000000..c760e55353bc
--- /dev/null
+++ b/drivers/dma-buf/dma-buf-sysfs-caps.c
@@ -0,0 +1,51 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * DMA-BUF sysfs capabilities.
+ *
+ * Copyright (C) 2022 Simon Ser
+ */
+
+#include <linux/kobject.h>
+#include <linux/sysfs.h>
+
+#include "dma-buf-sysfs-caps.h"
+
+static ssize_t sync_file_import_export_show(struct kobject *kobj,
+                                           struct kobj_attribute *attr,
+                                           char *buf)
+{
+       return sysfs_emit(buf, "1\n");
+}
+
+static struct kobj_attribute dma_buf_sync_file_import_export_attr =
+       __ATTR_RO(sync_file_import_export);
+
+static struct attribute *dma_buf_caps_attrs[] = {
+       &dma_buf_sync_file_import_export_attr.attr,
+       NULL,
+};
+
+static const struct attribute_group dma_buf_caps_attr_group = {
+       .attrs = dma_buf_caps_attrs,
+};
+
+static struct kobject *dma_buf_caps_kobj;
+
+int dma_buf_init_sysfs_capabilities(struct kset *kset)
+{
+       int ret;
+
+       dma_buf_caps_kobj = kobject_create_and_add("caps", &kset->kobj);
+       if (!dma_buf_caps_kobj)
+               return -ENOMEM;
+
+       ret = sysfs_create_group(dma_buf_caps_kobj, &dma_buf_caps_attr_group);
+       if (ret)
+               kobject_put(dma_buf_caps_kobj);
+       return ret;
+}
+
+void dma_buf_uninit_sysfs_capabilities(void)
+{
+       kobject_put(dma_buf_caps_kobj);
+}
diff --git a/drivers/dma-buf/dma-buf-sysfs-caps.h b/drivers/dma-buf/dma-buf-sysfs-caps.h
new file mode 100644
index 000000000000..d7bcef490b31
--- /dev/null
+++ b/drivers/dma-buf/dma-buf-sysfs-caps.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * DMA-BUF sysfs capabilities.
+ *
+ * Copyright (C) 2022 Simon Ser
+ */
+
+#ifndef _DMA_BUF_SYSFS_CAPS_H
+#define _DMA_BUF_SYSFS_CAPS_H
+
+struct kset;
+
+int dma_buf_init_sysfs_capabilities(struct kset *kset);
+void dma_buf_uninit_sysfs_capabilities(void);
+
+#endif // _DMA_BUF_SYSFS_CAPS_H
diff --git a/drivers/dma-buf/dma-buf-sysfs-stats.c b/drivers/dma-buf/dma-buf-sysfs-stats.c
index 2bba0babcb62..09e43c8891d6 100644
--- a/drivers/dma-buf/dma-buf-sysfs-stats.c
+++ b/drivers/dma-buf/dma-buf-sysfs-stats.c
@@ -141,21 +141,13 @@ static const struct kset_uevent_ops dmabuf_sysfs_no_uevent_ops = {
        .filter = dmabuf_sysfs_uevent_filter,
 };

-static struct kset *dma_buf_stats_kset;
 static struct kset *dma_buf_per_buffer_stats_kset;
-int dma_buf_init_sysfs_statistics(void)
+int dma_buf_init_sysfs_statistics(struct kset *kset)
 {
-       dma_buf_stats_kset = kset_create_and_add("dmabuf",
-                                                &dmabuf_sysfs_no_uevent_ops,
-                                                kernel_kobj);
-       if (!dma_buf_stats_kset)
-               return -ENOMEM;
-
        dma_buf_per_buffer_stats_kset = kset_create_and_add("buffers",
                                                            &dmabuf_sysfs_no_uevent_ops,
-                                                           &dma_buf_stats_kset->kobj);
+                                                           &kset->kobj);
        if (!dma_buf_per_buffer_stats_kset) {

You can drop the braces now.  Otherwise,

Reviewed-by: Jason Ekstrand <jason.ekstrand@xxxxxxxxxxxxx>

I've updated my Mesa MR to use this so someone should go review that patch so we have userspace.  I've tested everything and it works nicely together.
 
-               kset_unregister(dma_buf_stats_kset);
                return -ENOMEM;
        }

@@ -165,7 +157,6 @@ int dma_buf_init_sysfs_statistics(void)
 void dma_buf_uninit_sysfs_statistics(void)
 {
        kset_unregister(dma_buf_per_buffer_stats_kset);
-       kset_unregister(dma_buf_stats_kset);
 }

 int dma_buf_stats_setup(struct dma_buf *dmabuf)
diff --git a/drivers/dma-buf/dma-buf-sysfs-stats.h b/drivers/dma-buf/dma-buf-sysfs-stats.h
index a49c6e2650cc..798c54fb8ee3 100644
--- a/drivers/dma-buf/dma-buf-sysfs-stats.h
+++ b/drivers/dma-buf/dma-buf-sysfs-stats.h
@@ -8,9 +8,11 @@
 #ifndef _DMA_BUF_SYSFS_STATS_H
 #define _DMA_BUF_SYSFS_STATS_H

+struct kset;
+
 #ifdef CONFIG_DMABUF_SYSFS_STATS

-int dma_buf_init_sysfs_statistics(void);
+int dma_buf_init_sysfs_statistics(struct kset *kset);
 void dma_buf_uninit_sysfs_statistics(void);

 int dma_buf_stats_setup(struct dma_buf *dmabuf);
@@ -18,7 +20,7 @@ int dma_buf_stats_setup(struct dma_buf *dmabuf);
 void dma_buf_stats_teardown(struct dma_buf *dmabuf);
 #else

-static inline int dma_buf_init_sysfs_statistics(void)
+static inline int dma_buf_init_sysfs_statistics(struct kset *kset)
 {
        return 0;
 }
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 5e1b0534b3ce..b5c5a5050508 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -30,6 +30,7 @@
 #include <uapi/linux/dma-buf.h>
 #include <uapi/linux/magic.h>

+#include "dma-buf-sysfs-caps.h"
 #include "dma-buf-sysfs-stats.h"

 static inline int is_dma_buf_file(struct file *);
@@ -1546,22 +1547,54 @@ static inline void dma_buf_uninit_debugfs(void)
 }
 #endif

+/* Capabilities and statistics files do not need to send uevents. */
+static int dmabuf_sysfs_uevent_filter(struct kobject *kobj)
+{
+       return 0;
+}
+
+static const struct kset_uevent_ops dmabuf_sysfs_no_uevent_ops = {
+       .filter = dmabuf_sysfs_uevent_filter,
+};
+
+static struct kset *dma_buf_kset;
+
 static int __init dma_buf_init(void)
 {
        int ret;

-       ret = dma_buf_init_sysfs_statistics();
+       dma_buf_kset = kset_create_and_add("dmabuf",
+                                          &dmabuf_sysfs_no_uevent_ops,
+                                          kernel_kobj);
+       if (!dma_buf_kset)
+               return -ENOMEM;
+
+       ret = dma_buf_init_sysfs_capabilities(dma_buf_kset);
        if (ret)
-               return ret;
+               goto err_kset;
+
+       ret = dma_buf_init_sysfs_statistics(dma_buf_kset);
+       if (ret)
+               goto err_sysfs_caps;

        dma_buf_mnt = kern_mount(&dma_buf_fs_type);
-       if (IS_ERR(dma_buf_mnt))
-               return PTR_ERR(dma_buf_mnt);
+       if (IS_ERR(dma_buf_mnt)) {
+               ret = PTR_ERR(dma_buf_mnt);
+               goto err_sysfs_stats;
+       }

        mutex_init(&db_list.lock);
        INIT_LIST_HEAD(&db_list.head);
        dma_buf_init_debugfs();
        return 0;
+
+err_sysfs_stats:
+       dma_buf_uninit_sysfs_statistics();
+err_sysfs_caps:
+       dma_buf_uninit_sysfs_capabilities();
+err_kset:
+       kset_unregister(dma_buf_kset);
+       return ret;
 }
 subsys_initcall(dma_buf_init);

@@ -1570,5 +1603,7 @@ static void __exit dma_buf_deinit(void)
        dma_buf_uninit_debugfs();
        kern_unmount(dma_buf_mnt);
        dma_buf_uninit_sysfs_statistics();
+       dma_buf_uninit_sysfs_capabilities();
+       kset_unregister(dma_buf_kset);
 }
 __exitcall(dma_buf_deinit);
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index 70e213a0d7d9..ab3afd5da75a 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -114,6 +114,9 @@ struct dma_buf_sync {
  * ordering via these fences, it is the respnosibility of userspace to use
  * locks or other mechanisms to ensure that no other context adds fences or
  * submits work between steps 1 and 3 above.
+ *
+ * Userspace can check the availability of this API via
+ * /sys/kernel/dmabuf/caps/sync_file_import_export.
  */
 struct dma_buf_export_sync_file {
        /**
@@ -146,6 +149,9 @@ struct dma_buf_export_sync_file {
  * synchronized APIs such as Vulkan to inter-op with dma-buf consumers
  * which expect implicit synchronization such as OpenGL or most media
  * drivers/video.
+ *
+ * Userspace can check the availability of this API via
+ * /sys/kernel/dmabuf/caps/sync_file_import_export.
  */
 struct dma_buf_import_sync_file {
        /**
--
2.36.1



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